The construction of houses and other buildings involves the use of a variety of materials for walls, floorings and other surfaces. Solid hardwood or soft wood boards are highly desired for such surfaces, but solid boards are often prohibitively expensive. Veneer panels have often been used as an alternative for wall surfaces, but such panels pose their own concerns. As trees of the required type, size and quality become more and more scarce, the manufacture of multilayer veneers or plywoods is expensive with high quality veneer panels becoming difficult to obtain.
Gypsum boards or similar substrates are in widespread use as an alternative to solid boards or veneers. These synthetic panel stock materials are typically made from two outer layers of a thick paper material having an inorganic material, e.g., gypsum or calcium sulphate, in between. Gypsum board suffers from substantial loss of strength and/or structural integrity if the board becomes wet. Moreover, gypsum boards have no inherent grain structure so there is little inherent retention strength for nails, screws or the like which might be used for hanging paintings, photographs, ornaments, or shelving.
The competing needs of reasonable construction costs with high quality buildings has led to expanded uses for alternative wood products. For instance, particle board, fiber board, oriented strand board (OSB), hardboard, and other similar boards are formed from wood that may not otherwise be usable in the construction industry. Boards are also formed from particles, chips, flakes or other fragments of wood. These board stock are being used more and more in the construction of buildings, particularly for wall and floor surfaces and sub-surfaces. Such boards have a quality and integrity that is more than adequate for such uses.
Some of these alternative boards are vulnerable to swelling when exposed to moisture or water. These boards have been coated with wax or otherwise treated to avoid the problems with water. Lund U.S. Pat. No. 4,241,133 discloses that wood flakes may be bonded together with a binder. Examples of the binders include urea/formaldehyde resins, phenol/formaldehyde resins, melamine/formaldehyde resins and polyisocyanates. Binder concentrations of between 5 and 12% are disclosed. Waxes may be used for water resistance and preservatives may also be added. Other methods of manufacture of particle and similar boards are disclosed in U.S. Pat. Nos. 3,164,511 to A. Elmendorf; 3,391,233 to B. Polovtseff; and 3,940,230 to E. Potter.
Aminoplast resins like melamine-urea-formaldehyde (MUF) resins are used as a top spray on resin-containing wood fibers just before pressing the fibers into medium density hardboard. As the binder resin cures under heat and pressure, the board is provided with its structural properties. Simultaneously, the top spray resin cures and seals the surface with a hard protective coating. The following references disclose methods of preparing the thermoset resins used as the "The Chemistry of Synthetic Resins" by Carleton Ellis, Reinhold Publishing Co., 1935; "Phenolic Resin Chemistry" by N. J. L. Megson, Academic Press Inc., New York, 1958; "Aminoplasts" by C. P. Vale, Cleaver-Hume Press, Ltd., London, England; and British Pat. No. 480,316.
Often, a wood grain is molded into the board surface during the pressing step. There are times, however, when it is desirable to emboss a wood grain or other pattern into the surface of a finished board containing a top spray, such as MUF. See, Book U.S. Pat. No. 4,266,925 which is herein incorporated by reference. The embossing process involves the application of heat and pressure to the surface of the board which fractures the hard, brittle MUF coating. The resulting surface is unacceptable as well as weakening the cellulosic panel and rendering the surface vulnerable to humidity. Water extractable lignins will migrate to the surface through the fractures thereby causing surface discoloration and yellowing.
The present invention is directed to the embossing of cellulosic panels having a top spray coating that is otherwise too brittle to emboss without significant fracture. Specifically, the invention addresses a top spray coating and the use thereof in an embossing process that does not fracture the coating.
The brittleness of top spray coatings has been the subject of some concern in the art. Melamine-formaldehyde, urea-formaldehyde, and melamine-urea-formaldehyde polymers are often modified with glycols, sugars, and various latexes in attempts to reduce the brittleness of the thermosetting resins. Some attempts have been successful but at the cost of using modifying materials which may volatilize at embossing temperatures or otherwise migrate from the top coat to leave an uncured, low molecular weight residue on the surface of the press or embossing die surfaces. This buildup results in frequent nonproductive maintenance time for cleaning.
It would be useful to have a top coating that would be sufficiently flexible to accept embossing without materially affecting the hard thermosetting properties of the final coating or causing buildup on the press or embossing die surfaces.
The art has also investigated the embossing of panels having a basecoat finish on top of the formed panel. In these methods, a thermoplastic or organic solvent-based thermosetting basecoat is generally applied to the panel before embossing. Unfortunately, the conventional basecoats soften at press temperatures that are high enough to get good embossing at reasonable pressures, typically over 300.degree. C. and 5-9 Mpa. Pieces of the basecoat separate and stick to the embossing die surface when disengaged from the panel surface. The resulting product panel thus exhibits a surface having irregular areas lacking a basecoat and an interrupted finish. Such panels are unsuitable for further finishing.
Films have been used in attempts to prevent embossed basecoatings from sticking and separating. A thin film of a heat resistant material having a thickness of 0.5 to 1.5 thousandths of an inch, (e.g., Mylar.TM. film) is positioned between the embossing die and the board having the basecoat. Films can be effective at eliminating separation of the print basecoat and (depending on its thickness) does permit at least some limited amount of fine line wood grain detail to be retained. Films require, however, an extra handling step in the manufacturing process and cannot be reused. Such extra costs are often not worth the limited degree of detail obtained.
Resin-impregnated paper overlays are also known as protective media that might be used to accept embossed details. Such overlays come in medium and high densities and are basically made of paper containing partially cured phenol-formaldehyde resin. The paper overlays are bonded to a wooden panel using heat and pressure. Unfortunately and in addition to the handling costs for sheets, the exposed surface of the overlay is also sufficiently reactive that a release agent is required to prevent the resin from sticking to the embossing dies.
It would be desirable to have a method for embossing cellulosic panels that does not foul the press or embossing die surfaces or require the use of either separator sheets or embossable resin-impregnated sheets.